1. Field of the Invention
This application relates to an exhaust system with an exhaust gas purifier. More particularly, this application relates to a four-cycle engine with such an exhaust system.
2. Description of the Related Art
FIG. 1 shows that the composition of exhaust gas emitted by a four-cycle engine is dependent on the composition of the combustion gas burned by the engine. More particularly, when the air fuel (A/F) ratio varies, the composition of exhaust gas also varies. In this example, as the A/F ratio becomes richer than 14.6, the amount of CO and THC emitted rapidly increase and as the A/F ratio becomes leaner, the amount of CO and THC emitted decreases at a relatively low rate. This type of threshold value is sometimes referred to herein as a theoretical air fuel ratio or stoichiometric ratio. In some cases, some exhaust gas components exhibit a counter trend to that of the CO and THC. For example, emission of NOx increases for leaner A/F ratios and decreases when the A/F ratio is richer.
Two techniques are known for reducing or otherwise controlling the concentration of certain uncombusted elements in exhaust gas. Such techniques and systems deploying them are sometimes referred to as exhaust gas purifying techniques or exhaust gas purifiers. FIGS. 2 and 3 illustrate an engine equipped with an exhaust gas purifier for reducing CO, HC, and NOx in the exhaust gas of the engine. The engine includes a three-way catalyst 31 located in the middle of an exhaust pipe 30. An oxygen sensor 32 is positioned on an upstream side (relative to the flow of exhaust gas in the exhaust pipe 30) of the three-way catalyst 31 to measure the concentration of oxygen in the exhaust gas. The measured oxygen concentration is used to estimate the engine's A/F ratio and to calculate an amount of fuel to be mixed with the air and provided to the engine such that the A/F ratio is kept close to the theoretical A/F ratio (e.g., 14.6), whereby emissions of CO, HC, and NOx are limited as shown in FIG. 3.
The oxygen concentration feedback technique described above maintains a desirable A/F ratio for removing CO, HC, and NOx from the exhaust gas, but disadvantageously limits engine power. Consequently, an operator of a motorcycle or other vehicle with a small displacement engine using this exhaust gas purifying technique may be dissatisfied with the vehicle's power. Also, this exhaust gas purifying technique generally requires the use of a fuel injection system, including a fuel pump, a fuel regulator, a controller, and the like, to control the A/F ratio. These components are required because, as shown in FIG. 3, if the A/F ratio varies from the theoretical A/R ratios by more than a very small amount, the emission of at least one component of exhaust gas to be controlled will greatly increase. These components increase the cost and complexity of the engine.
A second known exhaust gas purification system is illustrated in FIGS. 4 and 5. The engine includes first and second catalysts 35 and 36 in an exhaust pipe 30A, with the second catalyst 36 being downstream (relative to the flow of exhaust gas in the exhaust pipe 30A) of the first catalyst 35. A secondary air induction pipe 37 introduces air that has not burned in the engine (referred to herein as “secondary air”) into the exhaust pipe 30A between the first and second catalysts 35, 36. In this system, the first catalyst 35 reduces NOx in the exhaust gas emitted from the exhaust pipe 30A and the second catalyst 36 oxidizes, and thereby reduces, CO and HC in the exhaust gas emitted from the exhaust pipe downstream of the first catalyst.
The exhaust gas purifier of FIG. 4 allows the A/F ratio of the combustion gas burned in the engine to be richer than the theoretical A/F ratio because the secondary air introduced upstream of the second catalyst 36 enhances the oxidization therein of CO and THC. FIG. 5 shows that the A/F ratio of the engine is significantly richer than the theoretical A/F ratio, but the second catalyst oxidizes CO and THC as if the engine combusted a leaner A/F ratio, near the theoretical A/F ratio. A richer A/F ratio enhances the power of the vehicle compared to exhaust gas purifier of FIG. 2. If the engine is a small displacement engine, the vehicle equipped with exhaust gas purifier of FIG. 4 will be more responsive to the operator, and thus more enjoyable to ride. Also, the exhaust gas purifier of FIG. 4 can operate with a conventional carburetor.